1. Field of the Invention
This invention relates to a process for the detection of discontinuities on long workpieces, in particular on tubes, rods and bars, as well as to an apparatus for the execution of the process.
2. Background Information
German Laid Open Patent Application No. 26 05 405 discloses a process for the ultrasonic inspection of tubes to detect defects having various orientations, in which Lamb or plate waves are generated and travel several times around the circumference of the tube, and the attenuation of the circulating waves is evaluated. In this process, neither a rotation of the tube nor of the transducer around the tube is necessary, as is required with many current conventional processes. With this configuration, defects having almost any orientation can be detected. Since this is an integral process, however, the defect detection sensitivity is essentially determined by the surface quality of the test piece. For longitudinally-oriented defects, there are reflections from the defects. The reflected wave pulses may interfere with the circulating waves, and there is no assurance that these defects will necessarily be detected. Moreover, the interference-free detection of the signal reflected at a defect is desirable, since a pulse reflection process is usually more sensitive than an integral or direct transmission process.
Such a pulse reflection process using electrodynamically generated surface waves, which can be combined with a transmission process, is disclosed in DE-OS 32 18 453, and is used to examine the running surfaces of railroad rails. In that case, transducers are used which act both bidirectionally and unidirectionally. With bilaterally active transducers, a secure separation of the reflection indications from the indications of the primary circulating waves is not always possible, which is particularly true for long workpieces having a small diameter. Moreover, for example, in the examination of thin-walled tubes using plate waves, there is a danger of exciting external modes such as sound or noise modes, which also circulate several times around the test piece, which have a group speed different from the test mode, and whose indications cannot be distinguished from reflection indications. The use of unidirectional transducers, may make it possible to suppress the indications of primary circulating waves and only to evaluate reflected pulses; however, these transducers may require that test shots or pulse or other excitation in both circumferential directions be conducted one after another, and thus result in a lower test speed or in other words a greater time requirement to make a test. Moreover, the expense for electronic equipment, calibration and adjustment of such a test apparatus which works with unidirectional transducers is usually correspondingly high.
A transducer used for inspection with circulating waves which takes advantage of the destructive interference at the site of the receiver is disclosed in Patent Abstracts of Japan, P 253, Feb. 9, 1984, Vol. 8, No. 31. However, such an arrangement may only be used with difficulty for the inspection of the external surface of tubes, rods and bars, since it is often almost impossible to keep the destructive interference in a 180 degree orientation under the operating conditions of a manufacturing plant.
German Laid Open Patent Application No. 36 22 500 discloses a process in which, for the specified measurement of amplitudes of wave pulses circulating tangentially, the receiver signals are synchronized with burst signals of the same frequency, suitable duration and starting delay. A prerequisite for the proposed process, however, is that the precise position of the indication of the primary circulating waves is known, by means of the knowledge of the geometric orientation of transmitter and receiver, and the sound velocity of the circulating wave pulses. For this reason, the process of the prior art is not suitable for the recognition of reflection indications, since the defects or the discontinuities in the workpiece can be located at any point on the circumference of the test piece.